I have an M9 because it’s the only Beretta 92 with the classic 92 styling (that is, with the original straight dust cover). The M9A1 has subtle refinements beyond the rail (I’ll assume you read all that by now), but the M9 had all the core qualities that I wanted, and it’s a great shooter.
I have two main reservations about the M9:
1. The Grip: It lacks the subtle cutout under the beavertail that shortens the trigger reach. Almost all other 92s have this.
2. The Sights: I don’t care for dot-and-post sights, but I had an artist friend convert them to three-dot. It’s true that the front sight can’t be replaced (I prefer fiber-optic), but she repainted mine with fluorescent paint, and I am satisfied with that.
The rail wasn’t important to me, and the beveled mag well, additional grip checkering and specially coated magazines were mere nice-to-haves. I wanted a classic as my baseline Beretta 92.
That said, I wouldn’t mind adding an M9A1 and M9A3 to the arsenal.